There are two ways of erasing a RW disc. The soft method just erases (or overwrite) the boot or main sector (I don't really know how that sector is called) so it can be overwritten again and it takes less than a minute. The full method overwrites the whole disc and can take up to 20 minutes, depending of the capacity.
The bad thing is that a RW disc doesn't last long forever. There's a certain number of times to rewrite.